The fluids of your vehicle are as important for smooth and trouble free running as any other component that aids in the running of the vehicle. It doesn’t matter how powerful or smooth the motor in your car is, if there is no oil in it, the engine is nothing but a big hunk of metal. Then there is the coolant, the brake oil, the power steering fluid, and the transmission oil. And among those fluids that often get ignored is your transmission fluid.

The transmission oil in your car guarantees that the car runs smooth and trouble free. But the stakes increase multifolds when it’s a car with an automatic transmission. Properties of both manual and automatic gear oils vary vastly. The transmission oil in a manual car is almost like the engine oil with high viscosity. Manual gear oil is usually very thick. Its main purpose is to keep the parts in the gearbox housing lubricated so you don’t grind the cogs and gears to metal dust during driving. In bikes, and even in some older cars, your engine oil works both for lubricating the engine as well as the gearbox. In cars like the classic Mini, the engine and the manual transmission share the same sump.

But the composition of ATF (automatic transmission fluid) is quite different than the regular manual transmission oil. In cars with auto gearbox, the transmission fluid not only lubricates the parts inside (planetary gears, torque convertor, etc.) but it also actually makes the car move.

Automatic transmission cross section

A conventional automatic transmission (non-CVT) is a wonderful piece of engineering. There already are a few blogs here explaining the operations of a traditional automatic transmission. You should check them out if you want to learn more about auto gearboxes. But in this blog, we will talk about using the wrong kind of oil in your car’s automatic transmission and how it can be damaged.The conventional auto gearbox contains various clutches or clutch disks. These disks are stacked together and are usually collectively called clutch pack. The clutch pack of an auto car contains friction disks, separators and a piston/plunger to engage or disengage the disks. The disks inside are multilayered and have a little gap in them when the car is in neutral and free-wheeling. It is so that the engine can run freely without driving the wheels. Basically the gap helps to disconnect drive of the engine crank shaft from the wheels. But the moment you put the gearbox in drive, the plunger drops and the clutches engage/come together and form a single block with no gaps in between and lock the wheels. And now the wheels of your car are rotating along with the engine rotation (depending upon the ratio of the gear). This is, of course, an oversimplification of an otherwise quite a complicated piece of engineering.

Clutch Plates

Automakers design the auto gearboxes to run on a specific coefficient of friction. And the recommended ATF basically makes sure that the clutch pack has minimum slippage (ideally zero friction). So let’s say your car is supposed to have a certain ATF but you decided to go for nonspecific oil, there is a high chance of your clutch pack not behaving the way it was meant to behave. The slippage of clutch disks means there will be heat (excessive heat). And that much extra heat, that is more than the required operating temperature, is not good for the life of the transmission. Hence the need of having transmission oil coolers in many automatic cars that either have bigger engines/transmissions or do a lot of heavy lifting and/or work in extreme conditions. That constant slippage due to the incorrect ATF is going to shorten the life of your transmission.

Auto transmissions are pretty bulletproof generally. It is the heat that causes the damage over the period of time. The auto gearboxes make heat by design. And what kind ATF you are using plays an important part in extending or shortening the life of your car’s automatic transmission. ATF contains various additives that keep oxidation and corrosion under control. Auto manufacturers recommend a specific ATF because they know that that particular oil is going to now oil perform the best in most conditions, it will also protect the internal components of the transmission.

Similarly, if you use regular ATF in your car with CVT gearbox, that can lead to transmission failure as well. Although sometimes there is cross-compatibility, but one should be always consult the car’s manual for correct oil recommendations.